[Verse 1: Slim Shady]

from D12 – American Psycho Lyrics on Rap Genius

Meaning

Slim Shady = Eminem.
He came up with this alter ego when he was taking a dump back then and he used it for when he’s preforming with his group, D12, altough he used it a lot on his solo career too.
The idea was that each one of the 6 founding members would have an alternate persona to rap as, thus forming, as Eminem once said:

twelve motherfuckers in six different bodies

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